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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 29 Apr 2010 (Thursday) 08:48
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REAL focal length vs. ACTUAL focal length on lenses

 
Buckeye88
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Apr 17, 2011 21:48 as a reply to  @ post 10089421 |  #31

Wow, I just ran into this today and was quite surprised. Had to do lots of searching to find a thread that covered the topic, but I figured it had to be on here somewhere. :)

My 18-135mm @ 135mm compared to a 70-200 2.8 @ 135mm at ~10ft (close to MFD for 70-200), both on a 60D.

18-135mm @ 135

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70-200 @ 135
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If I trust the 70-200, I guess my 18-135 is short when up close. I'd call the difference substantial. At longer distances they match. I agree with the earlier post about wanting a chart or table of FL vs. subject distance if there's that much variation.

I started out thinking this was some kind of EF vs. EF-S difference and was really scratching my head.



  
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TheRisingArms
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Apr 18, 2011 02:19 |  #32

I think also the wider the focal range of the lens, the more substantial this effect becomes.


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melcat
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Apr 18, 2011 02:43 |  #33

The effect is called "focus breathing" - Google that (with the quotes, otherwise you get a lot of yoga hits).




  
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SkipD
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Apr 18, 2011 04:36 |  #34

Buckeye88 wrote in post #12242508 (external link)
My 18-135mm @ 135mm compared to a 70-200 2.8 @ 135mm at ~10ft (close to MFD for 70-200), both on a 60D.

......At longer distances they match. .....

As I have mentioned before in this thread and others, the focal lengths of camera lenses are measured at infinity focus only. Thus, two lenses of different design but the same marked focal length - even fixed focal length ("prime") lenses - when used at short focus distances may show different fields of views. This has absolutely nothing to do with the format of the camera.

Buckeye88 wrote in post #12242508 (external link)
..... I agree with the earlier post about wanting a chart or table of FL vs. subject distance if there's that much variation.

I do not recall ever seeing any charts of focal length variations of lenses at different focus distances.

Buckeye88 wrote in post #12242508 (external link)
I started out thinking this was some kind of EF vs. EF-S difference and was really scratching my head.

Hopefully you and others realize this is not the case. ALL lenses for SLR cameras are marked with their actual focal lengths and not some "equivalent" focal length values.

As I said above in this post, the camera format has nothing at all to do with the situation illustrated in post #31 above.


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atlrus
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Apr 18, 2011 05:53 |  #35

Buckeye88 wrote in post #12242508 (external link)
Wow, I just ran into this today and was quite surprised. Had to do lots of searching to find a thread that covered the topic, but I figured it had to be on here somewhere. :)

My 18-135mm @ 135mm compared to a 70-200 2.8 @ 135mm at ~10ft (close to MFD for 70-200), both on a 60D.

18-135mm @ 135
QUOTED IMAGE
IMAGE LINK: http://www.flickr.com …buckeyemark88/5​629087239/  (external link)
IMG_1711 (external link)

70-200 @ 135
QUOTED IMAGE
IMAGE LINK: http://www.flickr.com …buckeyemark88/5​629085219/  (external link)
IMG_1710 (external link)


If I trust the 70-200, I guess my 18-135 is short when up close. I'd call the difference substantial. At longer distances they match. I agree with the earlier post about wanting a chart or table of FL vs. subject distance if there's that much variation.

I started out thinking this was some kind of EF vs. EF-S difference and was really scratching my head.

Wow, this really blew my mind, what a gigantic difference. I can see the point of the OP. Makes it that much harder to pick a lens if you don't have plenty of experience with it and know exactly what you'd get.

And I don't see how the car engine example works in this case. It appears that most of us take the FL as a standard, so it'd be more like the speedometer showing 40mph on both v4 and v8 engines, but the v8 actually moving at 60mph.


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SkipD
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Apr 18, 2011 06:14 |  #36

atlrus wrote in post #12243984 (external link)
Wow, this really blew my mind, what a gigantic difference. I can see the point of the OP. Makes it that much harder to pick a lens if you don't have plenty of experience with it and know exactly what you'd get.

Do an experiment for me and the other readers. Take each of the lenses and aim them at a subject roughly the same 10-foot distance as in your photos. Put the lenses into manual focus mode (unless they have full-time manual focus capability) and run the the focus ring from stop to stop while looking at the 10-foot subject. Let us know which lens showed radically different subject sizes as you changed the focus.

I tried this with my 20D and my 16-35 f/2.8L (original version), 24-70 f/2.8L, and 70-200 f/2.8L IS lenses and saw very little subject size variation while rotating the focus ring through the full range. As a subject got really fuzzy (out of focus), it appeared that the image was larger than when in focus but I could tell by center-to-center spacing of multiple items at the subject's position that the image size wasn't really changing a noticeable amount.


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bohdank
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Apr 18, 2011 06:17 |  #37

The 70-200 F4IS and 70-200MKII at 200mm show different magnifications at around 20 feet when I tested them against each other for IQ. Can't recall which was shorter. I think it was the MKII.


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atlrus
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Apr 18, 2011 07:37 |  #38

SkipD wrote in post #12244034 (external link)
Do an experiment for me and the other readers. Take each of the lenses and aim them at a subject roughly the same 10-foot distance as in your photos. Put the lenses into manual focus mode (unless they have full-time manual focus capability) and run the the focus ring from stop to stop while looking at the 10-foot subject. Let us know which lens showed radically different subject sizes as you changed the focus.

I tried this with my 20D and my 16-35 f/2.8L (original version), 24-70 f/2.8L, and 70-200 f/2.8L IS lenses and saw very little subject size variation while rotating the focus ring through the full range. As a subject got really fuzzy (out of focus), it appeared that the image was larger than when in focus but I could tell by center-to-center spacing of multiple items at the subject's position that the image size wasn't really changing a noticeable amount.

Why would it matter when the end result will be like the photos above? After all, we are all after the in-focus shots :)


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SkipD
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Apr 18, 2011 07:57 |  #39

atlrus wrote in post #12244335 (external link)
Why would it matter when the end result will be like the photos above? After all, we are all after the in-focus shots :)

The question I have is which of your lenses (maybe even both) seem to vary the focal length with focal distance. None of my lenses seem to exhibit that characteristic.


Skip Douglas
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Buckeye88
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Apr 18, 2011 08:07 |  #40

SkipD wrote in post #12244034 (external link)
Do an experiment for me and the other readers. Take each of the lenses and aim them at a subject roughly the same 10-foot distance as in your photos. Put the lenses into manual focus mode (unless they have full-time manual focus capability) and run the the focus ring from stop to stop while looking at the 10-foot subject. Let us know which lens showed radically different subject sizes as you changed the focus.

I tried this with my 20D and my 16-35 f/2.8L (original version), 24-70 f/2.8L, and 70-200 f/2.8L IS lenses and saw very little subject size variation while rotating the focus ring through the full range. As a subject got really fuzzy (out of focus), it appeared that the image was larger than when in focus but I could tell by center-to-center spacing of multiple items at the subject's position that the image size wasn't really changing a noticeable amount.

I thought of that experiment afterwards too. Unfortunately, the 70-200 was borrowed and has to go back today.

atlrus wrote in post #12244335 (external link)
Why would it matter when the end result will be like the photos above? After all, we are all after the in-focus shots :)

By doing it at various distances, it's a way to tell when your lens starts to exhibit the behavior, without having to compare it to another lens.




  
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SkipD
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Apr 18, 2011 08:22 |  #41

Buckeye88 wrote in post #12244429 (external link)
I thought of that experiment afterwards too. Unfortunately, the 70-200 was borrowed and has to go back today.

I guess I goofed and suggested the experiment by replying to the wrong person. Oh well. :rolleyes:

Anyhow, I suspect it's the 18-135 that may be showing the greatest difference between the two you used in the photos. Just try the experiment with the 18-135. Set it at 135 mm (for starters), look at something at roughly ten feet from you and rock the focus ring from one end to the other looking for a change in the field (angle) of view while you do that. Then you could try it at other focal lengths for grins and giggles. I'd be interested in the results. You wouldn't be making images but just seeing the results in the viewfinder.


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REAL focal length vs. ACTUAL focal length on lenses
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